1. Field of the Invention
The present invention relates to an improved method for reprocessing plastic waste material into a form suitable for reuse in plastic processing machinery, such as, an extruder.
2. Description of the Prior Art
Extrusion operations are known in which a plastic resin, such as polyethylene, is extruded as a free standing structural member of a uniform cross-sectional profile such as plastic tubing or I beams, or is extruded as a curtain on a material such as a web of paper. In such operations, the extruder generates an extrudate of molten waste plastic material or resin of some desired cross sectional form when the extruder is first started and when the extruder is purged. Also, there are times when the apparatus receiving or removing the extrudate is temporarily disabled, but the operation of the extruder must continue to avoid degrading the polymer or resin. All of these operations generate extrudate of some form of water plastic resin that must be collected and disposed of or reused.
One of the methods used by the prior art to handle curtains of waste plastic materials has been to pass them into a trough where they accumulate as an amorphous mass and solidify into a "log" upon cooling. The logs are extremely cubersome, containing up to 400 to 500 pounds of material. They are not readily reprocessable because of their bulk, non-uniformity of shape, non-uniform physical characteristics (for example, no attempt is made to separate clear from pigmented polymers), and because the logs often collect dust, dirt and other impurities. These logs are usually disposed of by being buried or burned, even though their polymer content has value as secondary reclaimed polymer.
Another method and apparatus for reprocessing waste plastic material of which U.S. Pat. No. 3,192,293 is exemplary gathers two or more selvage plastic bands together to form an integrated strip of closely bunched selvage bands, and thereafter the strip is crimped along its length to form a unitary continuous corrugated ribbon. The plastic ribbon is then cut into a plurality of small segments of substantially the same size as standard new pellets used in the sheet molding apparatus.
Another method and apparatus used by the prior art has been to cool and solidify the plastic curtain-like extrudate into irregular sheets by quenching them in a trough of water and then passing them to an apparatus which crops the sheets, heats and re-extrudes the sheets in pellet form for reuse. A significant problem encountered with this method, however, has been the need to dry the pellets before reuse to drive off water entrapped during quenching. Another problem has been the difficulty of identifying and removing areas containing impurities. Still another problem in practice is the handling difficulty encountered when sections of the molten curtain weld together, overlap, or cool into irregular or corrugated masses.
Still another method used by the prior art has been to cool and solidify the curtain-like material by quenching the material in a trough of water, and then wrapping the resulting irregular sheets around a drum. The sheets are later unwound from the drums, slit into strands, heated, and chopped to form pellets. This method is cumbersome to operate and has the previously mentioned problems of driving off entrapped water and identifying and removing defects. Problems have also been encountered in wrapping sheets around a drum when the cooled sheets contain discontinuities, corrugations, irregularities, overlaps, or blobs of resin. It is also difficult to separate sheets of different color.